1. Field of the Invention
The present invention relates to a discharge lamp lighting circuit, and more specifically to a discharge lamp lighting circuit provided with protection circuit to prevent breakdown of a semi-conductor switching element use in a driving transformer.
2. Description of the Related Art
A discharge lamp, especially a rare-gas discharge lamp for use in various scanners and lighting devices is often lighted by a high frequency voltage obtained by switching a DC power source. The voltage waveform induced in a transformer is oscillated by a resonant circuit composed of an inductance of the transformer and a stray capacitance at the time of switching, and the voltage to be applied to the semi-conductor for driving and the secondary voltage of the transformer rise. Especially, when a rare-gas discharge lamp is not connected or not lighted, the load is light, and therefore the primary voltage of the transformer rises further thereby possibly destroying the semi-conductor for driving, and at the same time the secondary voltage of the transformer also rises further generating continuously a high voltage equivalent to the starting voltage, which may result in dielectric breakdown of the transformer.
In order to overcome the above problem, a discharge lamp lighting device has been disclosed in Japanese Patent Application Laid-open No. Hei 10-41081. The discharge lamp lighting device disclosed utilizes resonant oscillation for pulse lighting, stabilizes a power supply voltage thereby stabilizing luminance of a fluorescent lamp, and is provided with a protection means working when the fluorescent lamp is not connected or not lighted. The protection means functions such that a current flowing in the discharge lamp is detected by a lamp current detecting means thereby stopping the driving of the switching element when the discharge lamp is not connected or not lighted.
FIG. 5 is a circuit diagram of the conventional discharge lamp lighting device above described. The discharge lamp lighting device includes a voltage oscillation type inverter of one transistor (hereinafter referred to as inverter) and is connected to both electrodes of a DC power supply 1 which outputs a voltage Vi. A discharge lamp (hereinafter referred to as fluorescent lamp) 2 has a rare gas, such as xenon, filled therein as a discharge gas and has fluorescent material coated on the inner wall of its glass tube. An equivalent circuit, when the fluorescent lamp 2 is lighted, can be shown as a series circuit consisting of a resistor and an interelectrode capacitance. Referring to FIG. 5, the inverter comprises: a step-up transformer 11 having a step-up ratio of N and including a primary winding 11p and a secondary winding 11s; a switching element 12 (power MOSFET) connected to the primary winding 11p; a resonant capacitor 13 connected in parallel to the switching element 12; and a switching control circuit 3. A pair of electrodes 2a and 2b of the fluorescent lamp 2 are connected to the secondary winding 11s, that is, to respective output terminals of the inverter.
At a primary side of the step-up transformer 11, a series resonant circuit is formed by a primary inductance (inductance of the primary winding 11p), and by a sum of a capacitance of the resonant capacitor 13, an output capacitance Coss (not shown) of the switching element 12 and an interelectrode capacitance (not shown) of the fluorescent lamp 2 converted to the primary side. The series resonant circuit has its resonant cycle set to be shorter than an off-time Toff of the switching element 12. The off-time Toff is controlled to be constant always. The switching control circuit 3, which comprises a switching control IC 4 for the inverter, a plurality of resistors and a plurality of capacitors, is connected to a gate terminal of the switching element 12, and the switching element 12 is driven by a switching control signal outputted from an output terminal 4B of the switching control IC 4, whereby the inverter is operated. A voltage detecting circuit 5, which detects the voltage Vin of the DC power supply 1, is connected to the switching control circuit 3.
The structure and operation of a lamp current detecting circuit 6 and a protection circuit (comparator COMP provided in the switching IC 4) will be described hereafter. The lamp current detecting circuit 6 detects a lamp current flowing in a capacitor 41 by causing the lamp current to flow to a resistor 43 via a capacitor 42 thereby converting into a voltage, and the voltage is rectified by a diode 44, smoothed by a capacitor 45, divided by resistors 46 and 47, and inputted to a base of a transistor 48. A resistor 49 and a capacitor 50 are connected to a collector of the transistor 48, that is, to an output terminal 6A, and the resistor 49 has its other end connected to the DC power supply 1 thereby supplying a voltage to the collector of the transistor 48. When the lamp current is zero, the transistor 48 has a base voltage of zero and therefore is in an xe2x80x9coff statexe2x80x9d. Consequently, the capacitor 50 is charged by the DC power supply 1 via the resistor 49, and the voltage at the output terminal 6A increases and gets at a voltage equal to a power supply voltage Vin when a delay time Td (for example, 5 seconds) elapses, which is determined by the values of the resistor 49 and the capacitor 50. And, when the lamp current is flowing, the transistor 48 has its base supplied with a voltage and therefore is in an xe2x80x9con statexe2x80x9d, and the output terminal 6A has a voltage of zero.
The protection circuit comprises the aforementioned comparator COMP provided in the switching control IC 4. The comparator COMP has its non-inverting input terminal connected to the output terminal 6A of the lamp current detecting circuit 6 and has its inverting terminal supplied with a reference voltage. The reference voltage is lower than the power supply voltage Vin. An output of the comparator COMP is connected to a driver DB. The driver DB is controlled such that when the voltage at the non-inverting terminal of the comparator COMP is higher than the reference voltage, the output of the comparator COMP goes up to a high level so as to stop the operation of the driver DB, whereby the switching control signal is held at a low level causing the inverter to stop its operation, and such that when the voltage at the non-inverting terminal of the comparator COMP is lower than the reference voltage, the output of the comparator COMP goes down to a low level so as to have no impact on the operation of the driver DB causing the inverter to operate normally.
The operation of the circuit depending on the presence/absence of the lamp current will be described. Referring to FIG. 5, when the fluorescent lamp 2 is not connected or not lighted, the lamp current is zero, therefore when the delay time Td elapses, the voltage at the output terminal 6A of the lamp current detecting circuit 6, that is the voltage at the non-inverting input terminal of the comparator COMP, becomes equal to the power supply voltage Vin thereby causing the output of the comparator COMP to cease. At the very start of supplying power, the fluorescent lamp 2 connected is not lighted, and therefore the lamp current is zero, but due to the delay time Td of the lamp current detecting circuit 6 the driver DB does not cease its operation in the immediate wake of starting power supply, and if the fluorescent lamp 2 is lighted within the delay time Td, the inverter operates normally. That is to say, when the lamp current is flowing at the normal operation, the voltage at the output terminal 6A of the lamp current detecting circuit 6, that is the voltage at the non-inverting input terminal of the comparator COMP, is zero, whereby the output of the comparator COMP goes down to a low level and the inverter operates normally.
FIGS. 6A and 6B show a voltage VF between a source terminal S and a drain terminal D of the switching element 12, and an output voltage VINV of the inverter, respectively referring to when the fluorescent lamp 2 is lighted normally, and when the fluorescent lamp 2 is not connected or not lighted, where the horizontal axis represents a time, and the vertical axis represents the voltage VF and the output voltage VINF, and where TS is a time of one cycle, and Toff is a time for which the fluorescent lamp 2 is out. Referring to FIG. 6A, when the fluorescent lamp 2 is lighted normally, the voltage VF is approximately 200 V at a point A and the output voltage VINV is approximately 2000 V at a point B, and referring to 6B, when the fluorescent lamp 2 is not connected or not lighted, the voltage VF is approximately 500 V at the point A and the output voltage VINV is approximately 5000 V at the point B.
The discharge lamp lighting device disclosed in Japanese Patent Application Laid-open No. Hei 10-41081 has the following problem. When the fluorescent lamp 2 is not connected or not lighted, the voltage VF increases to approximately 500 V at the point A and the output voltage VINV increases to approximately 5000 V at the point B, exceeding the withstanding pressure of the switching element 12, which may result in destroying the switching element 12. To prevent the destruction, the switching element 12 is adapted to cease its operation after a predetermined time (5 seconds) when the discharge lamp 2 is not connected or not lighted, but a stress of a high voltage is applied to the switching element 12 and the step-up transformer 11 for the predetermined time, and therefore it may happen that the switching element 12 and the step-up transformer 11 suffer dielectric breakdown or insulation failure.
The present invention has been made in light of the above problem, and it is an object of the present invention to provide a discharge lamp lighting circuit including a protection circuit to prevent breakage of the circuit when a rare-gas discharge lamp is not connected or not lighted.
In order to achieve the object, according to a first aspect of the present invention, a discharge lamp lighting circuit comprises: a driving means to send out a signal for lighting a discharge lamp; a short-circuit protection means to protect the driving means when the discharge lamp is shorted; an open-circuit protection means to protect the driving means when the discharge lamp is not lighted; and a control means to control the driving means according to a signal sent from the short-circuit protection means and the open-circuit protection means. The open-circuit protection means of the discharge lamp lighting circuit is adapted to send out to the control means a signal for limiting a current flowing in the driving means to or below a predetermined value when the tube current is equal to or lower than a first value predetermined, and a signal for sequentially increasing a current flowing in the driving means up to a rated current when the tube current is higher than the first value and also is equal to or lower than a second value predetermined, and adapted to stop a driving signal sent from the control means when a tube current flowing in the discharge lamp has a value equal to or lower than the first value after a predetermined time.
According to a second aspect of the present invention, in the discharge lamp lighting circuit of the first aspect, the open-circuit protection means includes: a current detecting means to detect a tube current flowing in the discharge lamp; a current changing means to operate according to the outcome of detection by the current detecting means thereby changing the tube current flowing in the discharge lamp; and a minimum current detecting means to start its operation at a predetermined time after the outcome of detection by the current detecting means is determined. A signal for limiting the current flowing in the driving means to or below a predetermined value is sent by the open-circuit protection means to the control means according to the outcome of detection by the current changing means, and a signal for stopping a driving signal sent from the control means is sent by the open-circuit protection means to the control means according to the outcome of detection by the minimum current detecting means after a predetermined time.
According to a third aspect of the present invention, in the discharge lamp lighting circuit of the first aspect, the current changing means sends to the control means a signal for driving the driving means by changing at least one of a duty ratio and a frequency of a pulse signal sent out from the control means thereby increasing the tube current when the tube current is equal to or higher than the first value and also is equal to or lower than the second value, and a signal for stopping the tube current from increasing when the tube current reaches a predetermined value.
According to a fourth aspect of the present invention, in the discharge lamp lighting circuit of the first aspect, the driving means includes a DC power supply, a switching element to turn on and off a current flowing from the DC power supply, and a transformer having its primary winding connected in series to the switching element, and the current detecting means includes a means to convert into a DC voltage an AC voltage between the both ends of a resistor connected in series to a secondary winding of the transformer and the discharge lamp.
According to a fifth aspect of the present invention, in the discharge lamp lighting circuit of the first aspect, the short-circuit protection means sends out to the control means a signal for stopping the driving signal sent from the control signal when a tube current flowing in the discharge lamp exceeds a predetermined value according to the outcome of detection by the current detecting means.
Consequently, when the discharge lamp is not lighted or not connected, the current flowing in the driving means is reduced to a low level thereby limiting a voltage generated at the time of turning off a driving circuit, and when a current flowing in the discharge lamp is equal to or lower than the predetermined value, a pulse signal sent from the driving means is stopped after a predetermine time thereby preventing breakdown of the driving circuit.